Devices for the instantaneous measurement of signal frequencies have been known and used for some time. The conventional instantaneous frequency measurement (IFM) receiver is a radio frequency (RF) receiver used primarily in electronic warfare (EW). Its basic function is to measure the frequency of pulsed signals radiated from hostile radar.
Generally, it may be said that IFM receivers measure the frequencies of incoming RF signals utilizing interferometric techniques by detecting the phase shift magnitudes produced in multiple calibrated delay lines. For instance, the received RF signal is divided and simultaneously introduced into a non-delayed path and a delay line of known length. Since the phase differences between the delayed and non-delayed receiver paths are functions of the input signal frequency, conversion of the phase difference signals to video provides signals whose amplitudes are related to the phase delay. These video signals are delivered to an encoding network which makes amplitude comparisons of the signals and determines the numerical value of the frequency.
Examples of devices using the above-mentioned techniques can be found in the prior art. See, for example, U.S. Pat. No. 4,194,206, entitled INSTANTANEOUS FREQUENCY MEASUREMENT RECEIVER WITH CAPABILITY TO SEPARATE CW AND PULSED SIGNAL, issued on Mar. 18, 1980 to Tsui and U.S. Pat. No. 5,109,188 entitled INSTANTANEOUS MEASUREMENT RECEIVER WITH BANDWIDTH IMPROVEMENT THROUGH PHASE SHIFTED SAMPLING OF REAL SIGNALS, issued on Apr. 28, 1992 to Sanderson et al., each of which devices have a general construction, as described above.
IFM receivers are capable of covering very broad input bandwidths, i.e., tens of gigahertz (GHz), and providing a quick measure of the frequency of a single dominant input signal. One of the shortcomings of a conventional IFM receiver is that in order to achieve the desired level of accuracy, which is typically several megahertz (MHz), a series of parallel delay paths with increasingly larger delay values are used to perform a number of phase measurements. The larger delay values produce a better frequency measurement accuracy, but also result in a frequency ambiguity because the total delay in these additional paths will most likely exceed the cycle time of the highest frequency. By combining measurements from all of the various delay paths it is possible to remove the ambiguities and obtain the desired frequency measurement accuracy. However, this is all done at the increased expense of added hardware to process the additional channels.
Another principle disadvantage to conventional IFM receivers is that their performance degrades rapidly in the presence of multiple simultaneous signals. This occurs when two or more signals arrive at the receiver in an overlapping time interval. Under these circumstances, if one signal is very much stronger than the others, the stronger signal blocks reception of the weaker signals. Alternatively, if two or more overlapping signals are close in signal strength, the receiver may not be able to reliably measure any of the frequencies of the signals due to the cross-interference.
Other shortcomings of conventional IFM receivers are that they cannot provide amplitude information about the received signals and they generally have poor sensitivity to weaker signals. Thus, in order to measure signal strength or amplitude, other receivers, such as crystal video receivers are generally required in addition to the IFM receivers. The poor sensitivity of conventional IFM receivers stems from the fact that they have a wide instantaneous bandwidth. Accordingly, noise and interference proportional to that bandwidth cannot be prevented from corrupting their performance.
Compressive receivers, on the other hand, are capable of making more precise frequency measurements, and are more capable of handling multiple simultaneous input signals, than are IFM receivers. The function of a compressive receiver is to take an input frequency band consisting of multiple signals and noise and separate the individual components into an ordered time sequence proportional to their frequencies. Thus, compressive receivers perform a real time spectral analysis of the input signals in the input bandwidth. A disadvantage is that they are more limited in input bandwidth frequency than are IFM receivers, i.e., typically less than one GHz.
It is therefore an object of the present invention IFM compressive receiver to combine the advantages of an IFM receiver with that of a compressive receiver in order to provide a receiver having wide input bandwidth coverage combined with high frequency resolution and multiple simultaneous signal handling capability.
It is also an object of the present invention IFM compressive receiver to provide an IFM receiver capable of providing amplitude information about received signals.
It is further an object of the present invention to provide a receiver which can function as an interferometer for the purpose of making angle of arrival measurements on received signals.